1. Field of the Invention
The invention relates to a method and an apparatus for cleaning silver or gold plated costume jewelry by an electrolytic process.
Silver, gold and copper (group IB of the periodic table) are referred to as coinage metals. They have been widely used as coinage and jewelry metals. Besides the fact that these metals often occur in the uncombined state in nature, they are quite highly unreactive. For instance, silver has an oxidation potential of E.sup.o =0.7994 V (Ag.sup.+ +e.sup.- .fwdarw.Ag), so that it does not readily dissolve. The prevalent oxidation state of silver is +1. Upon exposure to air, it is not attacked by oxygen. Upon exposure to hydrogen sulfide or foods which contain sulfur, however, silver tarnishes readily. Tarnish is a thin film of Ag.sub.2 S, which dulls the otherwise lustrous, reflective properties of silver, and which forms in the reaction, EQU 4Ag+2H.sub.2 S+O.sub.2 .fwdarw.2AgS+2H.sub.2 O
It is often necessary, or at least desireable, to clean such surfaces. Jewelry and tableware are often plated with silver. The latter is usually plated with a layer several (hundred) microns thick, so that any type of cleaning method is suitable. Accordingly, silver-plated tableware is usually cleaned with harsh and abrasive chemicals, which remove the layer of tarnish as well as a few additional layers of pure metallic silver. Sterling silver is 92.5% silver and 7.5% copper. Silver jewelry metal contains 80% silver and 20% copper or nickel, where the latter is added to harden the alloy.
Costume jewelry, however, is plated with a very thin layer (at most 10 microns) of 0.999 silver. It is thus paramount that only the tarnish be removed, while the silver is retained on the plate.